Slot coupled, polarized, egg-crate radiator

FIELD OF THE INVENTION

This invention relates generally to radio frequency (RF) antennas, and more particularly to RF array antennas.

BACKGROUND OF THE INVENTION

As is known in the art, a radar or communications system antenna generally includes a feed circuit and at least one conductive member generally referred to as a reflector or radiator. As is also known, an array antenna includes a plurality of antenna elements disposed in an array in a manner wherein the RF signals emanating from each of the plurality of antenna elements combine with constructive interference in a desired direction.

In commercial applications, it is often desirable to integrate RF antenna arrays into the outer surfaces or “skins” of aircraft, cars, boats, commercial and residential structures and into wireless LAN applications inside buildings. It is desirable to use antennas or radiators which have a low profile and a wide bandwidth frequency response for these and other applications.

In radar applications, it is typically desirable to use an antenna having a wide frequency bandwidth. A conventional low profile, wideband radiator has been a stacked-patch antenna which includes two metallic patches, tuned to resonate at slightly different frequencies and supported by dielectric substrates. Thicker substrates (e.g., foams) are preferred in order to increase bandwidth, but there is a trade-off between bandwidth and the amount of power lost to surface waves trapped between the substrates. This trade-off places a restriction on the scan volume and overall efficiency of the phased arrays. Additionally, thick foams increase volume and weight, and absorb moisture which increases signal loss.

Surface waves produced in stacked-patch radiators have undesirable effects. Currents on a patch are induced due to the radiated space waves and surface waves from nearby patches. Scan blindness (meaning loss of signal) can occur at angles in phased arrays where surface waves modify the array impedance such that little or no power is radiated. The array field-of-view is often limited by the angle at which scan blindness occurs due to surface waves.

Waveguide radiators used in “brick” type phased array arrangements (i.e. the feed circuit and electronics for each antenna element is assembled in a plane perpendicular to the antenna radiating surface) do not suffer from internal surface wave excitation with scan angles which limits scan volume, but these waveguide radiators typically do not have a low profile or a wide bandwidth. In addition, individual waveguide radiators must be fabricated and assembled in a brick type architecture thus increasing costs and reducing reliability.

It would, therefore, be desirable to provide a low cost, low profile radiator with a wide bandwidth and a large scan volume which can be used with tile-based or brick-based array arrangements which can be used in land, sea, space or airborne platforms applications.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a low cost, wide bandwidth, linear or circularly polarized waveguide radiator in a tile array arrangement, meaning all feed networks and active electronics are stacked vertically within the unit cell boundary for each antenna element, without the undesirable surface wave effects normally found in stacked patch antennas.

It is a further object to provide a radiator which can assume arbitrary lattice arrangements such as rectangular, square, equilateral or isosceles triangular, and spiral configurations.

In accordance with the present invention, a radiator includes a waveguide having an aperture and a patch antenna disposed in the aperture and electromagnetically coupled to the waveguide. With such an arrangement, each radiating element and associated feed network are electro-magnetically isolated from a neighboring radiating element, thus eliminating internal surface wave excitation and therefore extending the conical scan volume beyond ±70°.

In accordance with another aspect of the present invention, an antenna includes an array of waveguide antenna elements, each element having a cavity, and an array of patch antenna elements including an upper patch element and a lower patch element disposed in said cavity. Such an arrangement provides a low cost, wide bandwidth, linear or circularly polarized waveguide radiator in a tile array arrangement, which in one embodiment includes feed networks and active electronics stacked vertically within the unit cell boundary for each antenna element.

In accordance with another aspect of the present invention, an antenna includes a first dielectric layer having a first plurality of patch antenna elements responsive to radio frequency signals having a first frequency, a first monolithic conductive lattice disposed adjacent to said first dielectric layer, a second dielectric layer comprising a second plurality of patch antenna elements responsive to radio frequency signals having a second different frequency, disposed adjacent to said first monolithic conductive lattice. A second monolithic conductive lattice is disposed adjacent to said second dielectric layer, and the first lattice and said second lattice form a plurality of waveguides, each waveguide associated with each of a corresponding first and second plurality of patch antenna elements. Such an arrangement provides a radiator which can assume arbitrary lattice arrangements such as rectangular, square, equilateral or isosceles triangular, and spiral configurations and a wide bandwidth, low-profile, slot-coupled radiator having the bandwidth of a stacked-patch radiator and the large scan volume of a waveguide radiator.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which:

FIG. 1 is a plan view of a stacked-patch egg-crate antenna according to the invention;

FIG. 2 is a cross sectional view of a stacked-patch egg-crate antenna;

FIG. 3 is a bottom view of an exemplary slot layer and feed circuit;

FIG. 4 is a cross sectional view of a radiating element included in a stacked-patch egg-crate antenna and associated feed system;

FIG. 5A is a Smith chart of the normal and de-embedded impedance loci of the stacked-patch egg-crate antenna in one embodiment according to the invention;

FIG. 5B is a graph of the return loss of the stacked-patch egg-crate antenna in one embodiment according to the invention; and

FIG. 6 is a three-dimensional cut away view, of a stacked-patch egg-crate antenna according to an alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a stacked-patch egg-crate antenna <b>10</b> and associated feed system <b>100</b>, here adapted for X-band, is shown to include an upper patch layer <b>12</b> disposed on an upper egg-crate layer <b>14</b>.

The upper patch layer <b>12</b> includes a plurality of patches <b>24</b><i>a</i>-<b>24</b><i>n </i>(generally referred to as upper patch <b>24</b>) which are arranged on a substrate or patch carrier <b>26</b>. The dimension of the upper patch <b>24</b> is a function of the frequencies used in conjunction with the radiator subsystem <b>110</b>. In one embodiment used for X-band frequencies the upper patches <b>24</b> have a dimension of 0.27λ by 0.27λ where λ is the design wavelength of the antenna <b>10</b>. It will be appreciated by those of ordinary skill in the art that the patches in the egg-crate radiator could be rectangular, circular or have any number of features to control radiation and mode excitation. Using techniques known in the art, an arbitrary sized and shaped upper patch layer <b>12</b> can be fabricated to fit a particular application, polarization requirement (e.g., linear or circular) and mounting surface.

The upper egg-crate layer <b>14</b> includes upper sidewalls <b>28</b> that define a plurality of upper waveguides <b>30</b><i>a</i>-<b>30</b><i>n </i>(generally referred to as upper waveguide <b>30</b>). The dimensions of upper waveguide <b>30</b> are determined by the size and spacing of the upper patches <b>24</b> and the height H_upperof the upper sidewalls <b>28</b>. In one embodiment, the upper waveguide <b>30</b> has an opening of 0.500 inches by 0.500 inches and a height of 0.0950 inches.

A lower patch layer <b>16</b>, which is disposed adjacent to a lower egg-crate layer <b>18</b>, is disposed adjacent to the upper egg-crate layer <b>14</b>. The egg-crate layers <b>14</b>, <b>18</b> form the structural support and the array of waveguide radiators. The lower egg-crate layer <b>18</b> is disposed adjacent to the associated feed system <b>100</b> which includes a slot layer <b>20</b> which is disposed adjacent to a feed circuit layer <b>22</b>. This arrangement combines the bandwidth of a stacked patch radiator with the isolation of a waveguide radiator in a single laminated structure without the need of physical RF interconnects with the slot layer <b>20</b> passing the electromagnetic signals from the feed circuit layer <b>22</b> into the antenna <b>10</b>. Additional layers of the RF circuitry (sometimes referred to as a tile array) below the feed circuit layer are not shown.

The lower patch layer <b>16</b> includes a plurality of patches <b>32</b><i>a</i>-<b>32</b><i>n </i>(generally referred to as lower patch <b>32</b> which are arranged on a lower patch carrier <b>34</b>). The dimension of a lower patch <b>32</b> is a function of the frequencies used in conjunction with the antenna <b>10</b>. In one embodiment used for X-band frequencies, the lower patches <b>32</b> have a dimension of 0.35λ by 0.35λ. Using techniques known in the art, an arbitrary sized and shaped lower patch layer <b>16</b> can be fabricated to fit a particular application and mounting surface. It should be noted that an adjustment of the height of the upper sidewalls <b>28</b> primarily influences the coupling between the upper and lower patches <b>24</b> and <b>32</b> thereby controlling the upper resonant frequency of the egg-crate radiator passband and the overall bandwidth.

The upper patch layer <b>12</b> and the lower patch layer <b>16</b> are preferably fabricated from a conventional dielectric material (e.g. Rogers R/T Duroid®) having 0.5 oz. copper layers which are fusion bonded on to each side of the dielectric.

The egg-crate layer <b>14</b> and the egg-crate layer <b>18</b> are preferably machined from aluminum stock which is relatively strong and lightweight. The egg-crate layers <b>14</b>, <b>18</b> provide additional structure to support the upper patch layer <b>12</b>, the lower patch layer <b>16</b>, the slot layer <b>20</b>, and the feed circuit layer <b>22</b>. It should be appreciated that the egg-crate layers <b>14</b>, <b>18</b> can also be fabricated by injection molding the basic structure and metalizing the structure with copper or other conductive materials.

The lower egg-crate layer <b>18</b> includes lower sidewalls <b>38</b> that define a plurality of lower waveguides <b>36</b><i>a</i>-<b>36</b><i>n </i>(generally referred to as lower waveguide <b>36</b>). The dimensions of a lower waveguide <b>36</b> is determined by the size and spacing of the lower patches <b>34</b> and the height H_lowerof the lower sidewalls <b>38</b>. Together, the upper and lower waveguides <b>30</b> and <b>36</b> operate electrically as if they were a single waveguide and eliminate the system limitations imposed by the internal surface waves.

The slot layer <b>20</b> which includes slots <b>66</b> which electro-magnetically couple waveguides <b>36</b><i>a</i>-<b>36</b><i>n </i>the feed circuit layer <b>22</b> to form an asymmetric stripline feed assembly. The asymmetric stripline feed assembly uses a combination of materials and feed circuit arrangement to produce proper excitation and maximum coupling to each slot <b>66</b> which passes electromagnetic signals to the antenna layers <b>12</b>-<b>18</b>. Together, the two assemblies (slot layer <b>20</b> and the feed circuit layer <b>22</b> and the antenna layers <b>12</b>-<b>18</b>) produce a thin (preferably 0.169 inches for the X-band embodiment.), light, mechanically simple, low cost antenna. Adjustment of the height of the lower sidewalls <b>38</b> primarily influences the coupling between the lower patches <b>32</b> and slots <b>66</b> thereby controlling a lower resonant frequency of the egg-crate radiator passband and the overall bandwidth.

The feed circuit layer <b>22</b> includes a conventional dielectric laminate (e.g., Rogers R/T Duroid®) and is fabricated using standard mass production process techniques such as drilling, copper plating, etching and lamination.

As the thickness of a conventional antenna with dielectric or foam substrates increases to enhance bandwidth, the angle at which the lowest order surface wave can propagate decreases thereby reducing efficient antenna performance over a typical phased array scan volume. However, the low profile, waveguide architecture of the stacked-patch egg-crate antenna <b>10</b> eliminates surface waves that are trapped between elements enabling increased bandwidth and scan volume performance (greater than ±70°) which are critical parameters for multi-function phased arrays.

Each cavity formed by the stacked, metallic upper egg-crate layer <b>14</b> and lower egg-crate layer <b>18</b> physically isolates each antenna element from all other antenna elements. The metallic sidewalls <b>28</b> and <b>38</b> of the cavity present an electrically reflecting boundary condition. In either transmit or receive mode operation, the electromagnetic fields inside a given stacked-patch egg-crate cavity are isolated from all other stacked-patch egg-crate cavities in the entire phased array antenna structure. Thus, internally excited surface waves are substantially reduced independent of cavity height, lattice geometry, scan-volume, polarization or bandwidth requirements.

The relatively thin, upper patch carrier <b>26</b> also serves as an integrated radome for the antenna <b>10</b> with the upper and lower egg-crate layers <b>14</b>, <b>18</b> providing the structural support. This eliminates the need for a thick or shaped radome to be added to the egg-crate radiator and reduces the power requirements for an anti-icing function described below.

Referring now to FIG. 2, further details of the structure of the antenna <b>10</b> and feed subsystem <b>100</b> are shown with like reference numbers referring to like elements in FIG. <b>1</b>. The upper patch layer <b>12</b> includes a copper layer <b>27</b> disposed on a lower surface of the upper patch carrier <b>26</b>. The upper patch layer <b>12</b> is attached to the upper surface of sidewalls <b>28</b> of the upper egg-crate layer <b>14</b> by attachment layer <b>44</b><i>a. </i>

The lower patch layer <b>16</b> includes a copper layer <b>50</b> disposed on the upper surface of the lower patch carrier <b>34</b> and a bottom copper layer <b>54</b> disposed on the bottom surface of the lower patch carrier <b>34</b>. The lower patch layer <b>16</b> is attached to the lower surface of sidewalls <b>28</b> of the upper egg-crate layer <b>14</b> by attachment layer <b>44</b><i>b</i>. The lower patch layer <b>16</b> is attached to the upper surface of sidewalls <b>38</b> of the lower egg-crate layer <b>18</b> by attachment layer <b>44</b><i>c. </i>

The attachment layers <b>44</b><i>a</i>-<b>44</b><i>d </i>preferably use Ni—Au or Ni-Solder plating. The Ni—Au or Ni-Solder plating is applied to the lower and upper egg-crates layers <b>14</b> and <b>18</b> and the etched copper egg-crate pattern on the lower and upper patch layers <b>12</b> and <b>16</b> using standard plating techniques. The entire egg-crate radiator structure is then formed by stacking layers <b>12</b>-<b>18</b> and re-flowing the solder. Alternatively layers <b>12</b>-<b>18</b> can be laminated together using conductive adhesive pre-forms as is known in the art.

A waveguide cavity <b>56</b> is formed by the upper and lower egg-crate layers <b>14</b>, <b>18</b>, which includes patches <b>24</b><i>a </i>and <b>32</b><i>a</i>. The metallic sidewalls <b>28</b>, <b>38</b> of the cavity formed by the upper egg-crate layer <b>14</b> and the lower egg-crate layer <b>18</b> present an electrically reflecting boundary condition to the electromagnetic fields inside the cavity, equivalent to a wave-guiding structure. The electromagnetic fields are thus internally constrained in each waveguide cavity <b>56</b> and isolated from the other waveguide cavities <b>56</b> of the structure. Preferably the cavity for each egg-crate is 0.5 inch×0.5 inch for an X-band system.

The feed subsystem <b>100</b> includes slot layer <b>20</b> and feed circuit layer <b>22</b>. Slot layer <b>20</b> includes metal layer <b>64</b> and support layer <b>68</b>. Metal layer <b>64</b> includes slots <b>66</b> which are apertures formed by conventional etching techniques. Metal layer <b>64</b> is preferably copper. Feed circuit layer <b>22</b> includes stripline transmission line layer <b>72</b> and a lower copper ground plane layer <b>78</b>, with carrier layer <b>76</b> and via's <b>74</b> connecting the upper copper layer <b>72</b> with stripline transmission line layers (not shown) below the lower copper ground plane layer <b>78</b>. Slot layer <b>20</b> and feed circuit layer <b>22</b> are joined with attachment layer <b>44</b><i>e</i>. The feed subsystem <b>100</b> is assembled separately and subsequently laminated to antenna <b>10</b> with attachment layer <b>44</b><i>d</i>. As described above attachment layer <b>44</b><i>d </i>uses either a low temperature solder or a low temperature electrically conductive adhesive techniques to join the respective layers. Layers <b>72</b> and <b>78</b> are preferably copper-fused to carrier layer <b>76</b> which is a conventional dielectric material (e.g. Rogers R/T Duroid®).

The aluminum egg-crate layers <b>14</b> and <b>18</b> form the waveguide radiator cavity <b>56</b> and provide the structural support for the antenna. When assembled with the feed subsystem, the two aluminum egg-crates layers <b>14</b> and <b>18</b> and carrier layers <b>26</b> and <b>34</b> form the antenna <b>10</b>. This assembly can be bonded to a tile array stack-up (described below in conjunction with FIG. 4) using a low temperature solder or, equivalently, a low temperature electrically conductive adhesive layer. Alternatively, the egg-crate ribs allow the antenna <b>10</b> and feed subsystem <b>100</b> to be mechanically fastened with screws or other types of fasteners (not shown) to the tile array cold plate (described below in conjunction with FIG. <b>4</b>). This alternative embodiment allows serviceability by disassembly of the antenna from the tile array to replace active components. This service technique is not practical for conventional foam based radiators.

Table 1 summarizes the radiator material composition, thickness and weight for an embodiment constructed as a prototype for an X-band system.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" rowsep="1">TABLE 1</entry></row></thead><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>RADIATING ELEMENT STACK-UP</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="1" colwidth="70pt" align="left" /><colspec colname="2" colwidth="56pt" align="left" /><colspec colname="3" colwidth="42pt" align="center" /><colspec colname="4" colwidth="49pt" align="center" /><tbody valign="top"><row>&lt;entry /&gt;&lt;entry /&gt;<entry>Thickness</entry>&lt;entry /&gt;</row><row><entry>Component</entry><entry>Material</entry><entry>(in.)</entry><entry>Weight (oz.)</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1" /></row><row><entry>Upper Patch layer 26</entry><entry>Rogers 3006</entry><entry>0.0100</entry><entry>0.00603</entry></row><row><entry>Attachment Layer 44a</entry><entry>Ni-Cu-Sn(60%)/</entry><entry>0.0009</entry><entry>0.00043</entry></row><row>&lt;entry /&gt;<entry>Pb(40%)</entry></row><row><entry>Upper Egg-crate 14</entry><entry>Aluminum</entry><entry>0.0950</entry><entry>0.03364</entry></row><row><entry>Attachment Layer 44b</entry><entry>Ni-Cu-Sn(60%)/</entry><entry>0.0009</entry><entry>0.00043</entry></row><row>&lt;entry /&gt;<entry>Pb(40%)</entry></row><row><entry>Lower Patch Layer 34</entry><entry>Rogers 3010</entry><entry>0.0005</entry><entry>0.00348</entry></row><row><entry>Attachment Layer 44c</entry><entry>Ni-Cu-Sn(60%)/</entry><entry>0.0009</entry><entry>0.00043</entry></row><row>&lt;entry /&gt;<entry>Pb(40%)</entry></row><row><entry>Lower Egg-crate 18</entry><entry>Aluminum</entry><entry>0.0250</entry><entry>0.00610</entry></row><row>&lt;entry /&gt;&lt;entry /&gt;<entry>Total: 0.138</entry><entry>Total: 0.0505</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

It should be noted that the stacked patch egg-crate antenna <b>10</b> including layers <b>12</b>, <b>44</b><i>a</i>, <b>14</b>, <b>44</b><i>b</i>, <b>16</b>, <b>44</b><i>c</i>, and <b>18</b> has no bonding adhesives in the RF path which includes the waveguide <b>56</b>, upper and lower patches <b>24</b> and <b>32</b>, and corresponding support layer. The absence of bonding adhesives in the RF path helps to reduce critical front-end loss. Front-end ohmic loss directly impacts radar or communication performance by increasing the effective antenna temperature, thus reducing antenna sensitivity and, ultimately, increasing antenna costs. In a conventional foam based stacked-patch radiator, mechanically reliable bonding adhesives introduce significant ohmic loss at microwave frequencies and above. Reliability is an issue as thickness of adhesives and controlling foam penetration becomes another difficult to control parameter in production. Furthermore, it is difficult to copper plate and etch foam structures in large sheets, and typically the foam sheets require a protective coating against the environment.

Returning to FIG. 2, in operation an RF signal is coupled from active layers (not shown) through via <b>74</b> to the feed circuit layer <b>22</b>. Preferably the stripline transmission line layer <b>72</b> is located closer to the slots <b>66</b> in slot layer <b>20</b> (e.g. 7 mils) than the ground plane layer <b>78</b> (25 mils) providing an asymmetric, stripline feed circuit in order to enhance coupling to the slots <b>66</b>. The asymmetric, stripline feed circuit layer <b>22</b> guides a radio-frequency (RF) signal between the via <b>74</b> and the stripline transmission line layer <b>72</b>. The RF signal is coupled from the stripline transmission line to the non-resonant slot <b>66</b>. The lower and upper metallic egg-crate layers <b>18</b> and <b>14</b> form an electrically cut-off (non-propagating fundamental mode) waveguide <b>56</b> for each unit cell. The lower patch <b>32</b> and upper patch <b>24</b> inside the waveguide <b>56</b> resonate the slot, waveguide cavity, and radiating aperture at two distinct frequencies providing wide band RF radiation into free space.

When viewed as a transmission line, each patch <b>24</b>, <b>32</b> presents an equivalent shunt impedance having a magnitude of which is controlled by the patch dimensions and dielectric constant of the patch carriers <b>26</b>, <b>34</b>. The shunt impedance and relative separation of the patches (with respect to the non-resonant slot) are adjusted to resonate the equivalent series impedance presented by the non-resonant slot, waveguide cavity and radiating aperture, thus matching to the equivalent impedance of free space. The transmission line stubs <b>83</b><i>a</i>-<b>83</b><i>d </i>(FIG. 3) present a shunt impedance to the circuit which is adjusted to center the impedance locus on the Smith Chart (FIG. <b>5</b>A).

The fringing electromagnetic fields of the slot, upper and lower patches <b>24</b>, <b>32</b> are tightly coupled and interact to provide the egg-crate antenna <b>10</b> with an impedance characteristic represented by curves <b>124</b>, <b>132</b>, (FIG. 5A) centered on the X-Band Smith Chart indicating the normal and de-embedded impedance loci respectively. As noted, the relative size and spacing between the patches <b>24</b>, <b>32</b> and slot <b>66</b> are adjusted to optimize coupling and, therefore, maximize bandwidth. The coupling between the non-resonant slot <b>66</b> and lower patch <b>32</b> primarily determines the lower resonant frequency, and the coupling between the upper patches <b>24</b> and lower patches <b>32</b> primarily determines the upper resonant frequency.

Referring to FIG. 3, the slots <b>66</b> of the slot layer <b>20</b> (FIG. 1) are shown superimposed over the feed circuit layer <b>22</b> (FIG. <b>1</b>). The feed circuit layer <b>22</b> includes a plurality of balanced-feed unit cells <b>80</b><i>a</i>-<b>80</b><i>n </i>(generally referred to as balanced-feed unit cell <b>80</b>). Each of the plurality of balanced-feed unit cells <b>80</b> includes four isolated, asymmetric (i.e., the stripline is not symmetrically located between the ground planes) stripline feeds <b>82</b><i>a</i>-<b>82</b><i>d </i>(generally referred to as stripline feed <b>82</b>), each feeding a non-resonant slot <b>66</b><i>a</i>-<b>66</b><i>d </i>respectively which is located above the stripline feeds <b>82</b><i>a</i>-<b>82</b><i>d</i>. Stripline feeds <b>82</b><i>a</i>-<b>82</b><i>d </i>include a corresponding transmission line stubs <b>83</b><i>a</i>-<b>83</b><i>d</i>. The slots <b>66</b><i>a</i>-<b>66</b><i>d </i>are located in the separate slot layer <b>20</b> (FIG. <b>1</b>). Mode suppression posts <b>92</b><i>a</i>-<b>92</b><i>n </i>are disposed adjacent to each stripline feeds <b>82</b><i>a</i>-<b>82</b><i>d </i>in a balanced-feed unit cell <b>80</b>. The mode suppression posts are preferably 0.0156″ (standard drill size) diameter plated-through-holes. The 4×4 array of FIG. 3 depicts the balanced feed arrangement, but it should be appreciated that an arbitrary sized array, lattice spacing, arbitrary lattice geometry (i.e., triangular, square, rectangular, circular, etc.) and arbitrary slot <b>66</b> geometry and configuration can be used (e.g., single, full length slot or two orthogonal slots).

The mode suppression posts <b>92</b><i>a</i>-<b>92</b><i>n </i>isolate each of the stripline feeds <b>82</b><i>a</i>-<b>82</b><i>d </i>in a balanced-feed unit cell <b>80</b>, and each balanced-feed unit cell <b>80</b> is isolated from the other balanced-feed unit cells <b>80</b>. Depending on the arrangement of the stripline feeds <b>82</b><i>a</i>-<b>82</b><i>d</i>, a linear, dual linear, or circular polarization mode of operation can be achieved. The balanced feed configuration presented in FIG. 3 can be operated in a dual-linear or circularly polarized system. Coupling is enhanced by the thin, high dielectric constant polytetrafluorethylene (PTFE) layer <b>68</b> of slot layer <b>20</b> and adjustment of the length and width of transmission line stubs <b>83</b><i>a</i>-<b>83</b><i>d </i>that extend beyond the non-resonant slot.

In one embodiment a feed layer includes the feed circuit layer <b>22</b> from layer <b>78</b> up to the ground plane layer <b>64</b> of the slot layer <b>20</b> (FIG. <b>2</b>). The feed circuit layer <b>22</b> includes stripline feeds <b>82</b> (FIG. 3) to provide an impedance transformation from the via <b>74</b> (nominally 25 ohms) to the slot <b>66</b> and egg-crate radiator <b>10</b> (nominally 10 ohms). This compact stripline feed configuration uses two short-section transformers (i.e. the length of each section is less than a quarter wavelength) that matches the input impedance of the via to the slot and egg-crate radiator impedance over a wide bandwidth. The length and impedance of each transformer section is chosen to minimize reflections between the via and the slot. A wider section (35-mils) of the stripline feed, the transmission line stub <b>83</b><i>a </i>extends beyond the center of the slot with respect to the narrower sections (30-mils, 21-mils, 15-mils) of the stripline feed <b>82</b>. The transmission line stub <b>83</b><i>a </i>provides a shunt impedance to the overall circuit including via <b>74</b>, stripline feed <b>82</b>, slot <b>66</b>, and egg-crate layers <b>14</b>, <b>18</b>, and its length and width are adjusted to center the impedance locus on the Smith Chart and minimize the magnitude of the reactive impedance component of the circuit.

The pair of co-linear slots <b>66</b><i>a</i>-<b>66</b><i>d </i>(FIG. 3) are provided to reduce cross-coupling at the intersection between the orthogonal pair of co-linear slots and to allow more flexibility in the feed circuit design. The upper PTFE layer <b>68</b> (here 5-mils thick) and lower PTFE layer <b>76</b> (here 25-mils thick) of the feed assembly preferably have a dielectric constant of approximately 10.2 and 4.5, respectively, which enhances coupling to the slot layer <b>20</b>. In addition, the choice of dielectrics <b>68</b> and <b>76</b> allows a balanced feed configuration preferably including four slots to fit in a relatively small unit cell at X-Band (0.52 in. base×0.60 in. alt.) and permits reasonably sized transmission line sections that minimize ohmic loss and comply with standard etch tolerance requirements.

The slots <b>66</b><i>a</i>-<b>66</b><i>d </i>(FIG. 3) are non-resonant because they are less than 0.5 (where represents the dielectric-loaded wavelength) in length over the pass band. The choice of non-resonant slot coupling provides two benefits in the present invention. First, the feed network is isolated from the radiating element by a ground plane <b>90</b> that prevents spurious radiation. Second, a non-resonant slot <b>66</b> eliminates strong back-lobe radiation (characteristic of a resonant slot) which can substantially reduce the gain of the radiator. Each stripline feed <b>82</b> and associated slot <b>66</b> is isolated by 0.0156″ diameter plated through-holes. Table 2 summarizes the asymmetric feed layer material composition, thickness and weight.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" rowsep="1">TABLE 2</entry></row></thead><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>FEED LAYER STACK-UP</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="1" colwidth="56pt" align="left" /><colspec colname="2" colwidth="63pt" align="left" /><colspec colname="3" colwidth="49pt" align="center" /><colspec colname="4" colwidth="49pt" align="center" /><tbody valign="top"><row><entry>Component</entry><entry>Material</entry><entry>Thickness (in.)</entry><entry>Weight (oz.)</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1" /></row><row><entry>Upper Board 68</entry><entry>Rodgers RO3010;</entry><entry>0.005</entry><entry>0.00348</entry></row><row>&lt;entry /&gt;<entry>ε = 10.2, tanδ =</entry></row><row>&lt;entry /&gt;<entry>.003</entry></row><row><entry>Adhesive 44e</entry><entry>FEP; ε = 2.0, tanδ =</entry><entry>0.001</entry><entry>0.0010 </entry></row><row>&lt;entry /&gt;<entry>.0005</entry></row><row><entry>Lower Board 76</entry><entry>Rodgers TMM4; ε =</entry><entry>0.025</entry><entry>0.0114 </entry></row><row>&lt;entry /&gt;<entry>4.5, tanδ = .002</entry><entry>Total: 0.031</entry><entry>Total: 0.0159</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

Tanδ is the dielectric loss tangent and ∈ is the dielectric constant.

The balanced, slot feed network is able to fit in a small unit cell area: 0.52″ (alt.)×0.60″ (base). The height is thin (0.031″) and lightweight (0.0159 oz.). Coupling is enhanced between the stripline feed <b>82</b> and slot layer <b>20</b> by placing a thin (5-mil), high dielectric constant (10.2) PTFE sheet layer <b>68</b>, which concentrates the electric field in that region between the two layers <b>82</b> and <b>20</b>.

Preferably, standard etching tolerances (±0.5 mils for 0.5 oz. copper) and a low plated through-hole aspect ratio (2:1) are used. Wider line widths reduce ohmic losses and sensitivity to etching tolerances.

Alternatively the radiator design of the present invention can be used with a low temperature, co-fired ceramic (LTCC) multilayer feed. Slot coupling permits the egg-crate radiator to be fabricated from materials and techniques that differ from materials and construction of the slot layer <b>20</b> and feed circuit layer <b>22</b>.

Referring to FIG. 4, an X-Band tile-based array <b>200</b> includes an egg-crate antenna <b>10</b>, an associated feed subsystem <b>100</b>, a first Wilkinson divider layer <b>104</b>, a second Wilkinson divider layer <b>106</b>, a transformer layer <b>108</b>, a signal trace layer <b>110</b>, a conductive adhesive layer <b>112</b>, and a conductor plate <b>114</b> stacked together. Layers <b>104</b>-<b>106</b> are generally referred to as the signal divider/combiner layers. The X-band tile based array <b>200</b> further includes a coaxial connector <b>116</b> electrically coupled the connector plate.

The antenna <b>10</b> and feed subsystem <b>100</b> can be mechanically attached by fasteners to the active modules and electrically attached through a fuzz-button interface connection as is known in the art.

The Wilkinson divider/combiner layers <b>104</b> and <b>106</b> are located below the feed circuit layer <b>22</b> and provide a guided electromagnetic signal to a corresponding pair of co-linear slots <b>66</b><i>a</i>-<b>66</b><i>d </i>(FIG. 3) in-phase to produce an electric field linearly polarized and perpendicular to the pair of slots. Similarly, the second Wilkinson divider/combiner layer combines the signals from the orthogonal pair of co-linear slots. The resistive Wilkinson circuits provide termination of odd modes excited on the patch layers and thus eliminate parasitic resonances.

To produce signals having a circular polarization balanced feed configuration (FIG. <b>3</b>), a stripline quadrature hybrid circuit (replacing the transformer layer <b>108</b>) combines the signals from each Wilkinson layer in phase quadrature (i.e., 90° phase difference). The balanced slot feed architecture realizes circular polarization, minimizes unbalanced complex voltage excitation between the stripline feeds (unlike conventionally fed two-probe or two-slot architectures), and therefore reduces degradation of the axial ratio figure of merit with scan angles varying from the principal axes of the antenna aperture.

To produce signals having linear polarization, one pair of co-linear slots is removed and one slot replaces the other pair of co-linear slots. A single strip transmission line feeds the single slot thus realizing linear polarization.

Now referring to FIG. 5A, a Smith Chart <b>120</b> includes a curve representing the normal impedance locus <b>124</b> at via <b>74</b> (FIG. 2) on the feed layer and de-embedded impedance locus <b>132</b> de-embedded to slot <b>66</b> (FIG. 2) of the stacked-patch egg-crate antenna <b>10</b>.

Now referring to FIG. 5B, a return loss curve <b>134</b> illustrates the return loss for the entire stacked-patch egg-crate antenna <b>10</b> and associated feed system <b>100</b>. The return loss curve <b>134</b> represents the reflected power of the feed circuit layer <b>22</b> and slot layer <b>20</b> and stacked-patch egg-crate antenna <b>10</b> with the via input <b>74</b> terminated in a 25 ohm load. A return loss below a −10 dB reference line <b>138</b> (i.e., 10 percent reflected power) indicates the maximum acceptable return loss at the via input <b>74</b> (FIG. <b>2</b>). Curve <b>136</b> represents the effect of a low pass Frequency Selective Surface (described below in conjunction with FIG. <b>6</b>).

A heater is optionally incorporated into the upper egg-crate layer <b>14</b> (FIG. 1) by running a heater wire (not shown) in the egg-crate layer <b>14</b> to prevent ice from building up in the upper patch layer <b>12</b> or radome. An embedded anti-icing capability is provided by the upper egg-crate structure <b>14</b>. A non-conductive, pattern plated egg-crate, formed by conventional injection mold, photolithography and plating processes (e.g., copper or aluminum), includes a conductive cavity (for the radiator function) and a wire pattern (of suitable width and resistivity) plated to the upper face. Alternately, conductive metal wires made of Inconil (a nickel, iron, and chromium alloy) can be embedded between the upper egg-crate surface and upper patch carrier <b>26</b> (FIG. <b>1</b>). Insulated wires and a grounding wire are disposed in conduits in the lower and upper egg-crate ribs supplying power to the wire pattern at one end and a return ground at the other end. The resistive wire pattern generates heat for the upper patch carrier <b>26</b> to prevent the formation of ice without obstructing the waveguide cavities or interfering with radiator electromagnetic performance in any manner, for any given lattice geometry and for arbitrary polarization. The widths of the egg-crate ribs (20-mils and 120-mils in the present embodiment) accommodate a wide range of wire conductor widths and number of wires that allow use of a readily available voltage source without the need for transformers.

The upper patch <b>24</b> is etched on the internal surface of the upper patch layer <b>12</b>, which also serves as the radome, and protects the upper (and lower) patch from the environment. The lower and upper egg-crates provide the structural support allowing the upper patch layer to be thin (0.010 in. thick) thus requiring less power for the anti-icing grid, reducing operating and life-cycle costs and minimizing infrared radiation (thereby minimizing detection by heat sensors in a hostile environment). In contrast to a thick, curved radome, the thin flat radome provided by the upper patch layer significantly reduces attenuation of transmitted or received signals (attenuation reduces overall antenna efficiency and increases noise power in the receiver) and distortion of the electromagnetic phase-front (distortion effects beam pointing accuracy and overall antenna pattern shape). Overall, the egg-crate radiator architecture is low profile, lightweight, structurally sound and integrates the functions of heater element and radome in a simple manufacturable package.

Now referring to FIG. 6, an alternative embodiment includes a frequency selective surface (FSS) <b>140</b> having a third egg-crate layer <b>150</b> with a thin, low-pass FSS patch layer <b>152</b> disposed on the third egg-crate layer <b>150</b> in order to further reduce the radar cross section (RCS).

The FSS patch layer <b>152</b> preferably includes a plurality of cells <b>154</b><i>a</i>-<b>154</b><i>n </i>(generally referred to as cell <b>154</b>). Each cell <b>154</b> includes patches <b>156</b><i>a</i>-<b>156</b><i>d </i>which in this embodiment act as a low pass filter resulting in a modified return loss signal as indicated by curve <b>136</b> (FIG. <b>5</b>B). It will be appreciated by those of ordinary skill in the art that the size and number of patches <b>156</b> can be varied to produce a range of signal filtering effects.

Additionally the upper patch carrier <b>26</b> substrate can also accommodate integrated edge treatments (e.g., using PTFE sheets with Omega-ply® layers integrated into the laminate) that reduce edge diffraction. The fabrication techniques and materials used for a modified antenna would be similar. The tapered edge treatments act as RF loads for incident signals at oblique angles exciting surface currents that scatter and diffract at the physical edges of the antenna array. The upper egg-crate can also serve as the heater element and the low-pass frequency selective surface <b>140</b> can serve as the radome.

In still another embodiment, optically active materials are integrated in to the upper and lower patch layers <b>12</b> and <b>16</b>. The egg-crate ribs serve as the conduits to run fiber optic feeds (and thus eliminate any interference with the electromagnetic performance of the egg-crate radiators) to layer(s) of optically active material sheets bonded to either or both of the lower and upper egg-crates. The fiber optic signal re-configures the patch dimensions for instantaneous tuning (broad bandwidth capability) and/or presents an entirely “metallic” antenna surface to enhance stealth and reduce clutter. Silicon structures fabricated from a standard manufacturing process (and doped with an appropriate level of metallic ions) have demonstrated “copper-like” performance for moderate optical power intensities. In this embodiment of the egg-crate antenna <b>10</b>, a thin Silicon slab (doped to produce polygonal patterns when excited), would be placed on top of the lower and/or upper patch dielectric layers. When optically activated, the polygonal patterns become “copper-like” parasitic conductors tuning the copper patches on the lower and/or upper patch dielectric layers and thus instantaneously tuning the egg-crate cavity.

Another advantageous feature of the present invention is frequency scalability of the egg-crate radiator architecture without changing material composition or construction technique while still performing over the same bandwidth and conical scan volume. For example, the following Table 3 summarizes the changes in the egg-crate radiator dimensions scaled to the C-band (5 GHz) for the same material arrangement as shown in FIG. <b>2</b>.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="2"><colspec colname="1" colwidth="56pt" align="left" /><colspec colname="2" colwidth="161pt" align="center" /><thead><row><entry namest="1" nameend="2" rowsep="1">TABLE 3</entry></row><row><entry namest="1" nameend="2" align="center" rowsep="1" /></row><row><entry>Component</entry><entry>Dimension</entry></row><row><entry namest="1" nameend="2" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry>Upper Patch</entry><entry>0.26λ × 0.26λ</entry></row><row><entry>Upper Egg-Crate</entry><entry>1.00 in. × 1.00 in. (opening) × 0.170λ (height)</entry></row><row><entry>Lower Patch</entry><entry>0.40λ × 0.40λ</entry></row><row><entry>Lower Egg-Crate</entry><entry>1.00 in. × 1.00 in. (opening) × 0.025λ (height)</entry></row><row><entry namest="1" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

In addition, slot coupling (in contrast to probe coupling) to the egg-crate radiator allows design freedom in choosing the egg-crate material and processes independent of the feed layer materials. For example, the egg-crates could be made from an injection mold and selectively metalized. Furthermore, the upper and lower patch carriers, layers <b>12</b> and <b>16</b> respectively, can use different dielectric materials. The slot coupled, egg-crate antenna <b>10</b> can be used in a tile array architecture or brick array architecture.

All publications and references cited herein are expressly incorporated herein by reference in their entirety.

Having described the preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims.